Ocular pharmacokinetics of cephalosporins using microdialysis

The purpose of this study was to delineate the ocular pharmacokinetics of cephalosporins and investigate the presence of peptide transporters in the retina. New Zealand albino rabbits were kept under anesthesia. A concentric microdialysis probe was implanted in the vitreous chamber and linear probe across the cornea in the aqueous humor. Isotonic phosphate buffer saline was perfused through the probes, and samples were collected every 20 min over a period of 10 hr. A 500 microg dose of cephalexin, cephazolin, and cephalothin was administered intravitreally. Inhibition experiments were carried out in vivo, using gly-pro and gly-sar. The vitreal half-lives of cephalexin, cefazolin, and cephalothin were 185.38 +/- 27.25 min, 111.40 +/- 17.17 min, and 146.68 +/- 47.52 min, respectively. Cephalexin generated higher aqueous humor concentrations compared to cefazolin. The pharmacokinetic parameters of cephalexin in the presence of gly-pro, i.e., AUC (44452.06 +/- 3326.55 microg x min/ml), clearance (0.0013 +/- 0.0004 ml/min) and vitreal half-life (825.12 +/- 499.95 min) were different from that of the control (14612.83 +/- 4036.47 microg x min/ml, 0.0036 +/- 0.0011 ml/min, and 187.96 +/- 65.12 min, respectively). Gly-pro did not inhibit cefazolin, and gly-sar showed no effect on the pharmacokinetics of both drugs. These studies indicate the involvement of a peptide carrier in the transport of cephalosporins across the retina. Although gly-pro inhibited the elimination of cephalexin from the vitreous, the effect of an alpha-amino group on peptide carriers was not clearly evident.

Management of diarrhoea in HIV-infected patients

Human immunodeficiency virus (HIV) infection can weaken the immune system causing its inability to combat opportunistic infections. Managing the complexity of these opportunistic infections has created a challenge for healthcare professionals. Our knowledge on the aetiological agents causing opportunistic infections in immunocompromised hosts has increased over the last decade. Diarrhoeal diseases are frequent complications associated with HIV-infected patients. For most of the causes of diarrhoea, the clinical signs are non-specific, and the laboratory diagnostic workup is neither easy nor fast. This review provides data on aetiological approaches of common diarrhoeal diseases including viral, microbacterial, parasitic, bacterial and fungal infections, and HIV enteropathy; diagnostic evaluation; and treatment of diarrhoea in HIV-infected patients. This article will be helpful for those who are in the practice of managing diarrhoea in such patients.

Autogenous osteochondral morselised grafts for full thickness osteochondral defects in the knee joints of pigs

The aim of this study is to firstly ascertain the survival of autogenously grafted morselised cartilage for full thickness osteochondral defects in knee joints of pigs. Secondly, it is to determine the quality of the grafted cartilage that survives and to score to it based on a recognised and tested system of indices and thirdly, to recognise, if any, the potential for reconstitution of the osteochondral junction. Two groups of five pigs were followed up for six and 12 weeks. Similar osteochondral defects were created in the medial condyles of both knees with the right medial femoral condyle defect filled with graft and the left used as control and filled with gel foam. At the end of the study period, an independent pathologist assessed the defects macroscopically and microscopically with an accepted and comparable histological scoring system. Macroscopically, there was better filling of the defect and restoration of bony contour in the grafted group compared to the control. Microscopically, at six weeks, filling of the defect, nature of predominant tissue, matrix staining and nature of cells all showed significantly better histological score than the control using the Mann-Whitney U test at the level of significance of p<0.05. At 12 weeks, in addition to the above, the reconstitution of osteochondral junction also showed a significantly better score. Comparing the test groups at six and 12 weeks, the reconstitution of the osteochondral junction was significantly better at 12 weeks. In conclusion, the autogenous osteochondral morselised graft persisted as mature hyaline cartilage with good histological score at six weeks with significantly better reconstitution of osteochondral junction occurring at 12 weeks. The use of morselised graft allows for the inclusion of bone graft which possibly allows for larger amounts of donor tissue and thus the possibility of treating larger defects. In the human model the donor site would be the non-weight bearing surfaces of the knee such as the intercondylar notch as described by Walgenbach A and Stone KR at the 1997 Annual Meeting of the American Academy of Orthopaedic Surgeons in San Francisco.

Binding of cosalane--a novel highly lipophilic anti-HIV agent--to albumin and glycoprotein

Cosalane is a potent inhibitor of HIV replication with multiple sites of action. The purposes of this study were to (a) determine the extent and nature of cosalane binding to mucin, alpha(1)-acid glycoprotein (AAG), plasma, and human (HSA) and bovine serum (BSA) albumin, and (b) determine the primary site(s) of cosalane binding to HSA. Plasma protein binding of cosalane was studied by a gel filtration technique. Cosalane binding to HSA was also determined in the presence of salicylic acid. Competitive inhibition studies were conducted using warfarin, digitoxin, and diazepam to determine the primary HSA binding site(s) of cosalane. The drug was bound extensively to HSA and BSA and required 500-550 moles to saturate 1 mole of protein. Stoichiometries of cosalane binding to alpha(1)-acid glycoprotein (AAG) and mucin were between 30 and 50 mol/mol of either glycoprotein. The binding isotherm deviated from a rectangular hyperbola, suggesting self-association of the ligand. Salicylic acid decreased cosalane binding to HSA by one order of magnitude. Inhibition studies of cosalane to HSA revealed that the compound binds primarily to warfarin site with a K(i) of 1.24 +/- 0.24 nM. In summary, cosalane binds extensively to serum albumins and to a lesser extent to both AAG and mucin.

Chemical stability, enzymatic hydrolysis, and nasal uptake of amino acid ester prodrugs of acyclovir

The objective of this work was to improve nasal absorption of relatively impermeable small drug molecules via an amino acid prodrug approach. Acyclovir was selected as a model drug. L-Aspartate beta-ester, L-lysyl, and L-phenylalanyl esters of acyclovir were synthesized to investigate their effectiveness in enhancing nasal absorption of acyclovir. A stability study was conducted in phosphate buffer under various pH conditions at 25 and 37 degrees C. Enzymatic hydrolysis in rat nasal washings and plasma was conducted at 37 degrees C. A rat in situ nasal perfusion technique was utilized in this investigation to examine the rate and extent of nasal absorption of amino acid prodrugs. The remaining analyte concentrations in the nasal perfusate were quantitated by reversed-phase high-performance liquid chromatography. The results revealed that the L-lysyl and L-phenylalanyl esters were less stable than L-aspartate beta-ester. The stability of all three esters decreased with increasing pH and temperature. L-phenylalanyl ester is highly susceptible to plasma esterases, with an in vitro half-life 1.33 min. The rat in situ nasal perfusion study revealed that the extent of nasal absorption of acyclovir, L-lysyl and L-phenylalanyl esters was not significant (p < 1%). L-Aspartate beta-ester was absorbed to the extent of approximately 8% over 90 min of perfusion at an initial drug concentration of 100 microM. Nasal absorption of L-aspartate beta-ester of acyclovir was inhibited by L-asparagine but not by a dipeptide glycylsarcosine (Gly-Sar). The enhancement of acyclovir nasal absorption from the L-aspartate beta-ester prodrug suggests that nasal uptake of this prodrug probably involves an active transport system.

Enhanced pulmonary delivery of insulin by lung lavage fluid and phospholipids

Pulmonary delivery appears to be the most promising non-parenteral route of insulin administration. In this work, we investigated the enhancement of insulin absorption in the presence of phospholipids and lung lavage fluid in vivo and in vitro. In-vitro experiments of insulin uptake by type II cells showed a significantly enhanced absorption in presence of lavage fluid, compared to various buffer preparations. The same trend was obtained with in-vivo studies of tracheal instillation of insulin. The incorporation of phospholipids as absorption enhancers in 1,2-dipalmitoyl phosphatidylcholine (DPPC) dispersion was compared to blank liposomes. A significantly higher blood glucose decrease was observed with a DPPC-insulin physical mixture compared to liposome, suggesting a possible effect of the phospholipid chain physical state on the insulin in-vivo absorption.

Nasal absorption of tyrosine-linked model compounds

The objective of this research is to explore the amino acid carrier-mediated pathway for nasal uptake of tyrosine-linked model compounds. These molecules were synthesized by ether linkage between the aromatic hydroxyl group of L-tyrosine and the terminal hydroxyl group of model compounds. An enzymatically stable linkage was utilized to link the model compounds to L-tyrosine so that the loss of the compound from perfusate signals absorption only without complication from metabolism. The rat in situ nasal perfusion technique was utilized in this investigation. The analyte concentrations remaining in the nasal perfusates were quantitated by a reversed-phase high-performance liquid chromatography. All L-tyrosine-linked model compounds were absorbed across the nasal mucosa (15-50% absorbed), and the extent of nasal absorption was concentration dependent. The apparent first-order rate constants of intact conjugate loss from the nasal perfusate were calculated. The Michaelis-Menten kinetic model was applied to fit the perfusion data. The Michaelis constant, K(m), and maximum or saturation rate of compound loss from the perfusate, V(max), were calculated from Lineweaver-Burk plots [e.g., K(m) and V(max) are 0.57 mM and 0.00515 micromol/min, respectively, for one of the model compounds, O-(4-carboxyphenylmethyl)-L-tyrosine]. The absorption characteristics of all these model compounds exhibited saturable kinetics, whereas their parent compounds, possessing different octanol-water partition coefficients, showed either no absorption or transport by passive diffusion. Nasal uptake of L-tyrosine conjugates was inhibited by L-tyrosine. The results from this study indicate that nasal absorption of L-tyrosine-linked compounds is probably through an amino acid transport system. Exploitation of the amino acid transport pathway may be utilized to improve nasal absorption of poorly permeable drugs.

Prodrug based optimal drug delivery via membrane transporter/receptor

The carrier-mediated absorption of drugs and prodrugs across epithelial and endothelial barriers is emerging as a novel trend in biotherapeutics. This review examines the important advances in this field in the past decade. The feasibility of drug absorption of the parent drug or the appropriately modified prodrug via these transporters is discussed in detail. Several successful examples of synthesis of prodrugs recognised by the targeted transporters are described. The applicability of this approach in translocating drugs across the almost impenetrable blood-brain barrier (BBB) has also been examined.

Ocular pharmacokinetics in rabbits using a novel dual probe microdialysis technique

Ocular infections involve delicate internal structures of the eye that often require treatment with antimicrobial agents. A major constraint to the study of ocular drug absorption from systemic administration is the inaccessibility of the vitreous for continuous serial sampling. A novel dual probe microdialysis technique has been employed in our laboratory, which will enable the delineation of complete ocular pharmacokinetics of a drug. New Zealand albino rabbits weighing 2--2.5 kg were used. The animals were kept under anesthesia throughout the experiment. A concentric probe was implanted in the vitreous chamber about 3 mm below the corneal scleral limbus. Simultaneously a linear probe was implanted in the anterior chamber across the cornea. Intraocular pressure (IOP) was measured using Schiotz tonometer. The total protein concentrations in the aqueous and vitreous humors were determined using the Bio-Rad protein assay method. The aqueous and vitreous elimination kinetics of fluorescein were studied after intravitreal and systemic administrations over a period of 10 hr. Microdialysis technique was also compared to the conventional direct sampling technique by determining the intravitreal kinetics of fluorescein. Results suggest that IOP reverted to normal within 2 hr after the implantation of the probes. The increase in the vitreal total protein concentration was not significantly different from the baseline. The increase in the aqueous total protein concentration was less than five times the basal concentration throughout the experiment. The blood-aqueous and blood-retinal barrier integrity was delineated by determining the permeability index for fluorescein and were found to be 9.48 +/- 4.25% and 1.99 +/- 0.66% for the anterior and vitreous chamber, respectively. The rate constant of penetration of fluorescein into the anterior chamber was found to be 8.48 +/- 1.33 x 10(-2) min(-1), which was significantly higher than into the vitreous i.e. 4.34 +/- 2.82 x 10(-2) min(-1). The terminal elimination rate constant of fluorescein from the anterior chamber (1.48 +/- 0.79 x 10(-2) min(-1)) was found to be similar to that of the plasma terminal elimination rate constant (1.57 +/- 0.25 x 10(-2) min(-1)), but significantly higher than from the vitreous (3.0 +/- 0.7 x 10(-3) min(-1)). The terminal vitreal elimination rate constant of fluorescein after intravitreal administration was found to be similar by both microdialysis (3.98 +/- 0.6 x 10(-3) min(-1)) and direct sampling (4.38 +/- 1.4 x 10(-3) min(-1)) techniques. In case of direct sampling technique the area under the vitreous concentration-time curve was higher compared to that obtained by the microdialysis technique. There was no breakdown of the blood ocular barriers as shown by a very small change in the intraocular fluid protein concentrations. This was also confirmed by the fluorescein kinetics, which were in accordance with the previous studies. IOP data suggests that the intraocular fluid dynamics were not affected and the animals stabilized within 2 hr after the implantation of the probes. Fluorescein data suggests that the vitreous compartment is surrounded by a tighter barrier compared to the anterior chamber. This technique appears to be more sensitive, reproducible and requires only one animal for the determination of entire ocular pharmacokinetic profile.

Visualization of a water-selective pore by electron crystallography in vitreous ice

The water-selective pathway through the aquaporin-1 membrane channel has been visualized by fitting an atomic model to a 3.7-A resolution three-dimensional density map. This map was determined by analyzing images and electron diffraction patterns of lipid-reconstituted two-dimensional crystals of aquaporin-1 preserved in vitrified buffer in the absence of any additive. The aqueous pathway is characterized by a size-selective pore that is approximately 4.0 +/- 0.5A in diameter, spans a length of approximately 18A, and bends by approximately 25 degrees as it traverses the bilayer. This narrow pore is connected by wide, funnel-shaped openings at the extracellular and cytoplasmic faces. The size-selective pore is outlined mostly by hydrophobic residues, resulting in a relatively inert pathway conducive to diffusion-limited water flow. The apex of the curved pore is close to the locations of the in-plane pseudo-2-fold symmetry axis that relates the N- and C-terminal halves and the conserved, functionally important N76 and N192 residues.

Increased nitrite and nitrate concentrations in sera and urine of patients with cholera or shigellosis

OBJECTIVES

Nitric oxide (NO) is an important regulator of cell function. In the intestine, NO regulates blood flow, peristalsis, secretion, and is associated with inflammation and tissue injury. The objectives of this study were to assess and compare the role of NO in cholera, a noninflammatory enteric infection, and in shigellosis, a bacterial inflammation of the colon.

METHODS

We determined serum and urinary concentrations of nitrite and nitrate during acute illness and early convalescence in 45 hospitalized children: 24 with cholera and 21 with shigellosis; 18 healthy children served as controls. Nitrite and nitrate concentrations were determined spectrophotometrically using Greiss reaction-dependent enzyme assay.

RESULTS

Serum nitrite and nitrate concentrations were significantly (p < 0.05) increased during acute illness compared to the early convalescence in both cholera and shigellosis. Urinary nitrite and nitrate excretions were significantly (p < 0.01) increased during acute disease in shigellosis, but not in cholera. Nitrite concentrations correlated with stool volume (r2 = 0.851) in cholera and with leukocytosis (r2 = 0.923) in shigellosis.

CONCLUSIONS

Both cholera and shigellosis are associated with increased production of NO, suggesting its pathophysiologic roles in these diseases.

Inhibition of a model protease--pyroglutamate aminopeptidase by a natural oligosaccharide gum from Hakea gibbosa

The purpose of this study was to investigate the effect of the oligosaccharide gum from Hakea gibbosa on the activity of a model protease enzyme pyroglutamate aminopeptidase (5-oxoprolyl peptidase; EC 3.4.19.3) and to elucidate the mechanism responsible for the decreased activity. Enzyme kinetic studies were conducted at 37 degrees C in 100 mM potassium phosphate buffer with 10 mM EDTA, 5% (v/v) glycerol, and 5 mM DTT (pH 8) for 15 min and were performed both in the presence and absence of the gum. Enzymatic activity was determined by a colorimetric assay using the specific substrate L-pyroglutamic acid beta-napthylamide. The enzyme kinetics was studied at various substrate and gum concentrations. The velocity of the reaction was determined by the amount of the product (beta-napthylamine) liberated at each substrate and gum concentration. The Ks and Vmax of the enzyme in the absence of the gum were 24.40+/-2.14 microM and 502.95+/-28.90 nmoles x min(-1) x mg protein(-1), respectively. As the concentration of the gum was gradually increased from 0.1 to 2%, the value of the Vmax decreased from 318.94+/-21.46 to 158.83+/-24.51 nmoles x min(-1) x mg protein(-1) while Ks increased from 17.42+/-4.6 to 63.03+/-1.89 microM. The mechanism for the inhibition of the enzyme by Hakea appeared to be a mixed-linear type (a type of non-competitive inhibition) as suggested from Hanes-Woolf, Dixon and Cornish-Bowden plots. The turnover number, kcat, calculated for the enzyme also decreased from 14.09+/-0.81 to 4.45+/-0.69 min(-1) as the concentration of the inhibitor was incrementally increased from 0 to 2% (w/v). The K(i) and alphaK(i) calculated from Dixon and Cornish-Bowden plots were found to be 0.31+/-0.11% (w/v) and 1.33+/-0.42% (w/v), respectively. The natural gum from Hakea gibbosa was effective in non-competitively inhibiting the enzyme pyroglutamate aminopeptidase. Thus, the natural gum may be a promising additive not only for its sustained-release and mucoadhesive properties as shown previously, but also for its ability to slow the enzymatic degradation of therapeutic polypeptides incorporated in dosage forms.

Prodrugs in nasal drug delivery

Prodrugs have been used to overcome poor solubility, insufficient stability, incomplete absorption across biological membranes and premature metabolism to active species. This review examines the importance of various physicochemical factors affecting nasal absorption of drugs. Novel trends in nasal prodrug development in the areas of targeted delivery to the CNS and selective targeting of the nutrient transporter system of the nasal mucosa have received considerable attention.

Risk factors and gender differentials for death among children hospitalized with diarrhoea in Bangladesh

To identify risk factors for death among children with diarrhoea, a cohort of 496 children, aged less than 5 years, admitted to the intensive care unit of a diarrhoeal disease hospital in Bangladesh, was studied during November 1992-June 1994. Clinical and laboratory records of children who died and of those who recovered in the hospital were compared. Deaths were significantly higher among those who had altered consciousness, hypoglycaemia, septicaemia, paralytic ileus, toxic colitis, necrotizing enterocolitis, haemolytic-uraemic syndrome, invasive or persistent diarrhoea, dehydration, electrolyte imbalances, and malnutrition. Females experienced a 2-fold higher risk of death than males (p = 0.003). Several indices of severe infections were identified more frequently among females than males. Females with severe infections were less frequently brought to the hospital than their male counterparts. The time lapse between onset of symptoms and hospital admission was significantly higher in females than males. This study suggests initiation of programmes to alleviate social disparity between genders for healthcare in poor communities. The study-results may also help physicians identify either prognostic indicators or risk factors for death among children hospitalized with severe illnesses associated with diarrhoea.

Lipid emulsions as vehicles for enhanced nasal delivery of insulin

The objective of this work is to explore lipid emulsion based formulations of insulin as an enhancer of nasal absorption. Insulin was incorporated into the aqueous phases of water-in-oil (w/o) and oil-in-water (o/w) emulsions. The formulations were perfused through the nasal cavity of rats in situ. Enhancement of insulin absorption was observed when insulin was incorporated into the continuous aqueous phase of an o/w emulsion. The presence of a small fraction of oil droplets along with insulin in the aqueous phase appeared to favor insulin absorption. When the oil phase constitutes the external phase, as in w/o emulsion, no insulin absorption was noted. Inhibition of insulin absorption might arise from a rate limiting barrier effect of the membrane completely covered by a stagnant oil layer. The in situ model was validated by in vivo experiments, which also revealed an increase in insulin absorption with o/w emulsions. However at lower insulin doses there was no statistically significant enhancing effect. In situ perfusion experiments across rat nasal pathway appear to be an appropriate model to study the enhancement effect of nasal formulations.

Effect of aging on tryptophan hydroxylase in rat brain: implications on serotonin level

Tryptophan hydroxylase (TrpH) catalyzes a rate-limiting step in the biogenesis of serotonin. The main objective of this study is to investigate the effect of aging on the activity of TrpH in serotonergic-enriched brain regions such as midbrain, pons, and medulla. TrpH activity was monitored by incubating various concentrations of tryptophan in a fixed amount of brain homogenate from midbrain, pons, and medulla of 2-month (young), 12-month (mature), and 24-month (old) rats (pH 7.4, 37 degrees C). The product 5-hydroxytryptophan was quantitated using a reversed phase HPLC equipped with an electrochemical detection system. Michaelis-Menton constants, K(m) and V(max), were calculated using the Lineweaver-Burk plot. The affinity (K(m)) of the enzyme significantly declined in midbrain and pons of old rats (141.1 +/- 2. 6, 126.0 +/- 10.8 microM) relative to mature rats (22.4 +/- 7.7, 38. 2 +/- 4.7 microM). However, no change was observed in medulla of old rats. The V(max) of TrpH in pons of all three age group rats was fairly constant. However, the V(max) of midbrain was significantly elevated, whereas that of medulla was reduced in old rats relative to mature rats. Clearance formation, a ratio of V(max) to K(m), of 5-hydroxytryptophan declined significantly in midbrain, pons, and medulla of old rats relative to mature rats. A combined effect of inefficient phosphorylation and oxidative damage of TrpH enzyme may be responsible for lower TrpH activity in aging brain. Such alterations in TrpH activity may reduce the level of serotonin in brain, which may be linked to late-life depression and other brain disorders, such as Alzheimer and Parkinson diseases.

Three-dimensional fold of the human AQP1 water channel determined at 4 A resolution by electron crystallography of two-dimensional crystals embedded in ice

Here, we present a three-dimensional (3D) density map of deglycosylated, human erythrocyte aquaporin 1 (AQP1) determined at 4 A resolution in plane and approximately 7 A resolution perpendicular to the bilayer. The map was calculated by analyzing images and electron diffraction patterns recorded from tilted (up to 60 degrees ), ice-embedded, frozen-hydrated 2D crystals of AQP1 in lipid bilayer membranes. This map significantly extends the findings related to the folding of the AQP1 polypeptide chain determined by us at a lower, 7 A by approximately 20 A, resolution. The solvent-accessible volume within a monomer has a vestibular architecture, with a narrow, approximately 6.5 A diameter constriction near the center of the bilayer, where the location of the water-selective channel is postulated to exist. The clearly resolved densities for the transmembrane helices display the protrusions expected for bulky side-chains. The density in the interior of the helix barrel (putative NPA box region) is better resolved compared to our previous map, suggesting clearer linkage to some of the helices, and it may harbor short stretches of alpha-helix. At the bilayer extremities, densities for some of the inter-helix hydrophilic loops are visible. Consistent with these observed inter-helix connections, possible models for the threading of the AQP1 polypeptide chain are presented. A preferred model is deduced that agrees with the putative locations of a group of aromatic residues in the amino acid sequence and in the 3D density map.

Kinetics of a model nucleoside (guanosine) release from biodegradable poly(DL-lactide-co-glycolide) microspheres: a delivery system for long-term intraocular delivery

The objective of this study was to prepare poly(DL-lactide-co-glycolide) (PLGA) microspheres containing guanosine as a model drug for intraocular administration. Microspheres were prepared by solvent evaporation technique using o/w emulsion system. The influence of composition and molecular weight of PLGA, drug loading efficiency, microsphere size, and in vitro and in vivo release rates were determined. Differential scanning calorimetry (DSC) and FTIR studies were conducted to examine the guanosine-polymer interaction. In vitro release studies indicated that the permeant release from microspheres exhibits an initial burst followed by slow first-order kinetics. Ascending molecular weights of the polymers generated progressively slower release rates. Three different sizes of microspheres were prepared. The release continued for 7 days with a maximum of 70% of the content released within that time period. DSC and FTIR studies showed no polymer-guanosine interaction. A novel microdialysis technique was used to examine the initial release kinetics from microspheres in isolated vitreous humor. This technique was also employed to observe in vivo intravitreal release in albino rabbits. A good correlation exists between in vitro and in vivo release rates from both 75 and 140 kDa PLGA microspheres. Guanosine-loaded microspheres could be prepared for once-a-week intravitreal injection with minimum required concentration maintained throughout the dosing interval. Because the structural and solubility characteristics of guanosine are similar to those of acyclovir and ganciclovir (two acycloguanosine analogues effective against herpes simplex virus [HSV-1] and cytomegalovirus [CMV], respectively), similar biodegradable polymer-based microsphere technology can be employed for the long-term intraocular delivery of these two drugs.

Transport of cosalane-a highly lipophilic novel anti-HIV agent-across caco-2 cell monolayers

Cosalane is a potent inhibitor of HIV replication with activity against a broad range of viral targets. However, the oral bioavailability of this highly lipophilic compound is extremely poor (<1%). Also, cosalane accumulates in high concentration in the liver after intravenous administration, with clear resistance to hepatic metabolism. In the present study, the transcellular permeability of cosalane was examined using Transwell(R) filter as well as plastic-grown confluent Caco-2 cell monolayers. A cell-culture-based biophysical model was adopted to understand the interactions of protein binding, membrane partitioning, and aqueous solubility of cosalane in limiting transcellular flux of cosalane across Caco-2 cell monolayers. The transcellular permeability (P(app)) of cosalane was extremely low (4.494 x 10(-8) cm/s) and the effect of p-glycoprotein on the efflux of cosalane was negligible. A characteristic disparity exists between the kinetics of cosalane uptake from apical (AP) donor solution and efflux into basolateral (BL) receiver side. The AP uptake of cosalane was rapid, exhibiting exponential kinetics, and reached equilibrium within 60 min, whereas the concomitant appearance of the compound into the BL receiver side was slow but linear over time. Furthermore, the uptake of cosalane was significantly reduced in the presence of bovine serum albumin (BSA). In unidirectional efflux studies, AP efflux of cosalane was limited in the absence of BSA. Also, no detectable metabolites were found in Caco-2 cell incubations. In conclusion, the present study demonstrates that diffusion of cosalane across Caco-2 cell monolayers is extremely limited and kinetically regulated essentially by the equilibrium between protein-bound and free drug partitioning into cell membrane.

Vitreal elimination kinetics of large molecular weight FITC-labeled dextrans in albino rabbits using a novel microsampling technique

A novel sampling technique that allowed for continuous vitreal sampling of high molecular weight compounds was developed. This technique generated consistent and reproducible results. Fluorescein isothiocyanate-linked dextrans (FITC-dextrans) with average molecular weights of 4.4, 9.3, and 38.9 kD were selected for the study. A 100 microgram dose was administered into the vitreous by a short-term infusion (100 microL) over a period of 45 s, and sampling was carried out for 10 h. The vitreal elimination of these dextrans was found to follow apparent first-order elimination kinetics, having half-lives of 246 min, 275 min, and 484 min, respectively. Aqueous levels were also determined at the end of 10 h and were correlated with vitreal dextran concentrations. The FITC-dextrans displayed an initial equilibration phase of about 200 min followed by linear first-order elimination. Apparent diffusion coefficients in the vitreous have been calculated to be 7.56 x 10(-6) and 6.18 x 10(-6) cm(2)/s for 4.4 and 9.3 kD dextrans, respectively. Furthermore, it became evident that with progressively higher molecular weight FITC-dextrans the vitreal elimination rate constant gradually decreased. The elimination rate constant was found to be inversely related to the logarithm of molecular weight with a correlation coefficient of 0.983. Results obtained suggest an elimination mechanism primarily involving the transretinal route possibly with some involvement of the aqueous pathway.

Polymorphism in the packing of aquaporin-1 tetramers in 2-D crystals

Hitherto, the packing arrangement of the aquaporin-1 (AQP1) tetramer in 2-dimensional (2-D) crystals (two-sided plane group p42(1)2) was observed to be largely similar (canonical crystal form) despite the difference in the source of the protein, the glycosylation state of the protein, the type of lipids, and the ratio of lipid to protein in the crystallization mixture. We report here our observation that the packing of AQP1 tetramers shows polymorphism in 2-D crystals generated in dioleoyl phosphatidylcholine bilayers. Apart from the canonical form, three additional allomorphs were identified. One was observed when small (0.25) lipid to protein ratio was used in the crystallization mixture while the other two were observed when the divalent cation content in the canonical crystals was modified. The various allomorphs were distinguished by different relative orientations of the AQP1 tetramer viewed in projection. The same, two-sided plane group p42(1)2 and similar unit cell dimensions were maintained in the different allomorphs as established by analysis of images of frozen-hydrated, nominally untilted crystals. Our results indicate that the interaction between the AQP1 monomers at the interface of the tetramers is flexible and is also strongly influenced by Mg(2+) ions with the cation effect materializing because of the intrinsic fluidity of the membrane.

Pharmacokinetics, biliary excretion, and tissue distribution of novel anti-HIV agents, cosalane and dihydrocosalane, in Sprague-Dawley rats

Cosalane and dihydrocosalane are potent inhibitors of HIV replication with a broad range of activity. The purpose of this study was to investigate: 1) the pharmacokinetic disposition of both cosalane and dihydrocosalane in male Sprague-Dawley rats, and 2) biliary excretion, enterohepatic circulation, and tissue distribution of cosalane after i.v. and/or oral administration. Animals were administered i.v. (10 mg/kg) cosalane or dihydrocosalane through a jugular vein to obtain plasma profiles. Dose dependence of cosalane was studied over a dose range of 1.0 to 10 mg/kg. The extent of enterohepatic recycling, biliary excretion, and tissue distribution were studied after i.v. administration. Both cosalane and dihydrocosalane exhibited a biexponential disposition with very long half-lives of 749 +/- 216 and 1016 +/- 407 min, along with very large volumes of distribution 23.1 +/- 4.4 and 24.4 +/- 2. 5 liter/kg, respectively. Both cosalane (nondetectable) and dihydrocosalane (<1%) showed very poor oral bioavailability. The biliary and renal excretions of cosalane were found to be negligible with no detectable metabolites either in urine or bile. After oral administration, more than 87% of the cosalane dose was excreted in the feces as the parent compound. Also, cosalane was sequestered significantly in liver with quantifiable levels in all tissues tested, even 48 h after the dose was administered. Therefore it was concluded that the poor oral bioavailability of cosalane may be due to its poor enterocytic transport coupled with sequestration in liver parenchymal cell membrane layers.